The present invention relates to transmission apparatus for two vehicle displacement members disposed one behind the other in the direction of displacement of the vehicle, the apparatus including a hydraulic pump and first and second hydraulic motors to which the first and second displacement members are respectively coupled, said hydraulic motors being dual-capacity motors, each of which comprises two elementary motors, each elementary motor having first and second elementary connections, for feed or discharge purposes, the first elementary connections being put in common to form a first main connection of the dual-capacity motor, while the second elementary connections are separate and form respectively second and third main connections of the dual-capacity motor, the first main connection of the first hydraulic motor being connected to a fluid feed duct coming from the pump and the first main connection of the second hydraulic motor being connected to a fluid return duct, the second main connections of the first and second hydraulic motors being interconnected via an interconnection duct.
The invention is applicable, for example, to vehicles having a hydraulic transmission with two displacement members disposed in line, one of which is steerable. For example such a vehicle may be a tandem compactor whose displacement members are constituted by rollers.
As explained below, the invention is also applicable to vehicles having two groups, each of two displacement members disposed in line, on either side of the vehicle. For example, such vehicles may be four-wheel-drive farm vehicles such as a combine harvester.
Patent Application EP 0 176 427 discloses three-port dual-capacity motors, and EP 0 547 947 applies them to a vehicle having three or four drivable wheels.
An object of the invention is to improve known apparatus so as to provide the vehicle with at least two speed ranges in translation, while continuing to make provision to synchronize the displacement members driven by the first and second hydraulic motors so as to prevent them from spinning.
This object is achieved by the fact that the apparatus of the invention further includes a selector having two or more positions and connected to said fluid feed duct, to said fluid return duct and to the third main connection of at least one of the first and second hydraulic motors, said selector having a high-torque position in which both hydraulic motors when as considered together with respect to how they are fed with fluid by the pump present a first total cylinder capacity, and a high-speed position in which both hydraulic motors when considered together with respect to how they are fed with fluid by the pump present a second total cylinder capacity smaller than said first total cylinder capacity.
It is known that, when two motors are fed in series, then, with respect to how they are fed with fluid, they present the same cylinder capacity that is equal to the cylinder capacity of one of the two motors. In contrast, when the same motors are fed in parallel, then, with respect to how they are fed with fluid by the pump, they present a large cylinder capacity that is equal to the sum of their respective cylinder capacities. When the cylinder capacity is high, the drive torque delivered by the motors is also high, which makes it possible for the vehicle to travel under difficult conditions, e.g. on a slippery building site. In contrast, when the cylinder capacity is low, the drive torque delivered by the motors is lower, and the vehicle can then move at a fast speed, under conditions different from its high-torque conditions.
Depending on which of its positions it takes up, the selector of the apparatus of the invention makes it possible to place certain elementary motors of the dual-capacity motors in series or in parallel, which, for the reasons indicated above, makes it possible to obtain a plurality of speed ranges.
It is possible to consider making provision for three types of configuration, namely a high-torque configuration, a medium configuration, and a high-speed configuration.
In the high-torque configuration, the third main connection of the first motor is connected to the fluid return duct while the third main connection of the second motor is connected to the fluid feed duct.
In the medium configuration, the third main connection of the first motor is connected to the third main connection of the second motor.
In the high-speed configuration, the third main connection of the first motor is connected to the fluid feed duct, while the third main connection of the second motor is connected to the fluid return duct.
The selector of the apparatus of the invention then has at least two positions corresponding respectively to one of the above-indicated configurations and to another one of said configurations.
In the high-torque configuration and in the high-speed configuration, the second main connections of the first and second hydraulic motors communicate with each other. In other words, the first elementary motors of each of the two hydraulic motors are disposed in series, so that the displacement members are synchronized.
In the high-torque configuration, the second elementary motors are connected in parallel relative to the series circuit of the first elementary motors. Therefore, with respect to how they are fed with fluid by the pump, the two motors have a total cylinder capacity equal to the sum of the cylinder capacity of the first motor fed first by the pump plus the cylinder capacity of the second elementary motor of the second motor, also fed by the pump.
In the medium configuration, the first and second elementary motors of the second hydraulic motor are connected in series respectively to the first elementary motor and to the second elementary motor of the first hydraulic motor. Therefore, the total cylinder capacity offered by the two motors with respect to how they are fed with fluid by the pump is equal to the entire cylinder capacity of the first hydraulic motor, i.e. to the sum of the cylinder capacities of the two elementary motors of the first hydraulic motor.
In the high-speed configuration, the two elementary motors of each of the two hydraulic motors are deactivated because their two connections (first main connection and second elementary connection) are connected to the same duct, i.e. to the fluid feed duct for the first hydraulic motor, and to the fluid return duct for the second hydraulic motor. The total cylinder capacity offered by both of the hydraulic motors with respect to how they are fed with fluid by the pump is equal to the cylinder capacity of the first elementary motor of the first hydraulic motor that is fed first by the pump, the first elementary motor of the second hydraulic motor being disposed in series.